FIG. 1 shows an optical system, which comprises an optical transmitter circuit 1 and an optical receiver circuit 3.
In the example considered, the transmitter circuit 1 comprises a signal generator (SG) 10 and transmission means 12 for generating an optical waveform, such as a LED (Light Emitting Diode) or a laser diode, e.g. operating in the infrared (IR) light region, i.e. the transmission means 12 may be an IR LED.
For example, the signal generator 10 may drive the transmission means 12 in order to transmit a pulse P by generating a corresponding light pulse, e.g. in the order of hundreds of ns (nanoseconds), e.g. between 100 and 500 ns.
The optical receiver circuit 3 comprises a light sensor PD, such as a photodiode, e.g. an IR photodiode, which is connected to a transimpedance amplifier (TIA) 32.
Generally, the light sensor PD is arranged to receive the light generated by the transmission means 12.
Accordingly, in the example considered, the transimpedance amplifier 32 converts the current provided by the photodiode PD into a corresponding voltage signal Vout being indicative of the intensity of light received by the photodiode PD.
In the example considered, a processing circuit (PC) 36, which generally may be any analog and/or digital circuit, such as a micro-processor, e.g. a DSP (Digital Signal Processor), may elaborate the voltage signal Vout in order to detect the light pulse in the received signal.
Generally, also further analog and/or digital signal processing (SP) stages 34 may be provided between the transimpedance amplifier 32 and the processing circuit 36, such as one or more amplifier stages and/or filters, such as bandpass filters, and/or an analog-to-digital converter.
FIG. 2 shows a second example of a receiver circuit 3.
Specifically, in the example considered, the optical receiver circuit 3 comprises a plurality of light sensors, such as three photodiodes PD1, PD2 and PD3, e.g. IR photodiodes, which are connected through switching means 30 to the input of the same transimpedance amplifier 32.
Generally, the switching means 30 may be configured to connect a subset of the light sensors PD1, PD2 and PD3 to the input of the transimpedance amplifier 32. For example, in various applications, at each instant, only a single light sensor PD1, PD2 and PD3 is connected to the input of the transimpedance amplifier 32. For example, these multiple photodiodes PD1, PD2 and PD3 may be placed in a given area in order to “map” the space with a proper resolution. In fact, each photodiode can provide a related spatial information by reading a certain TX pulse amplitude. In order to minimize the number of receivers (area, consumption, etc.) several photodiodes PD1, PD2 and PD3 can be multiplexed on the same receiver. In this case only one photodiode will be connected to the receiver at each time.
Accordingly, in the example considered, the transimpedance amplifier 32 converts the current provided by the photodiode PD currently connected to the input of the transimpedance amplifier 32 into a corresponding voltage signal Vout being indicative of the intensity of light received by the respective photodiode(s) PD.
FIG. 2 shows also a possible connection of the photodiodes PD1, PD2 and PD3 to the input of the transimpedance amplifier 32. Specifically, in the example considered, each of the photodiodes PD1, PD2 and PD3 is connected with its cathode to a constant voltage VPD and with its anode via the switching means 30 to the input of the transimpedance amplifier 32. For example, the switching means 30 may comprise a respective electronic switch SW1, SW2 and SW3 for each of the photodiodes PD1, PD2 and PD3, which permit to:                connect the anode of a respective photodiode PD1, PD2 or PD3 to the input of the transimpedance amplifier 32, or        disconnect the anode of the respective photodiode PD1, PD2 or PD3 from the input of the transimpedance amplifier 32.        
Accordingly, in the example considered, the photodiodes PD1, PD2 and PD3 (when connected through the switch to the transimpedance amplifier input virtual ground) are biased in the reverse region and will thus produce a background current (also called dark current), i.e. each of the photodiodes PD will produce also a current, when none of the transmission means 12 generates a light pulse. For example, the amplitude of the input current signal generated by the light pulse may generate a current variation in the range of 1-2 uA (microampere), which is significantly smaller than the background current, which is often in the range of hundreds of uA.
Generally, a similar problem may also arise due to ambient light. In fact, in many applications the current provided to the input of the transimpedance amplifier 32 will have an DC offset on which is modulated some kind of signal, e.g. the effect of the pulse P shown in FIG. 1.
Accordingly, after a settling time, in which the amplifier 32 should be able to cancel this DC-like background current, the transmission diode 12 can generate a light pulse that will be received by the photodiode PD and converted into a proportional current pulse, which is fed to the input of the TIA 32.
For example, as shown in FIG. 3, a transimpedance amplifier circuit 32 with DC offset cancellation may be implemented with a transimpedance amplifier 320.
Specifically, in the example considered, the amplifier circuit 32 comprises an input IN for receiving an input current IIN and an output OUT for providing an output voltage Vout. In the example considered, the input current IIN is fed to the input of the transimpedance amplifier 320, which thus generates the output voltage Vout at the output.
The output voltage Vout is also fed to feedback control loop comprising an integrating error amplifier 324, which generates via a current source 328 a compensation current IDC, which is also fed to the input of the transimpedance amplifier 320, i.e. the transimpedance amplifier 320 receives at input a current ITIA corresponding to:ITIA=IIN+IDC  (1)
Specifically, the error amplifier 324 generates a feedback control signal for the current source 328 by comparing the output voltage Vout with a reference voltage Vref. Accordingly, in the example considered, the error amplifier 324 will vary the current IDC provided by the current source 328 until the output voltage Vout corresponds to the reference voltage Vref.
In the example considered, low-pass (LP) filters 322 and/or 326 may be arranged at the input and/or the output of the error amplifier 324, respectively, thereby using only the DC offset as basis for the generation of the compensation current IDC at the input of the transimpedance amplifier 320.
Reference is also made, for example, to United States Patent Application Publication No. 2004/0119539 (incorporated by reference) which discloses various solutions of a transimpedance amplifier with DC offset cancellation.
There is a need in the art to provide solutions for improving the DC/low frequency offset cancellation at the input of a transimpedance amplifier.